Epilepsy is a common disorder, affecting approximately 1.3 million women of child-bearing age in the United States. Seizures during pregnancy can cause increased risks to both the mother and fetus. These risks have to be balanced against the known teratogenic effects of antiepileptic drugs (AEDs). During pregnancy, the sex steroid hormones estradiol and progesterone increase dramatically. Sex steroid hormones and the metabolic byproducts that are capable of modifying neural activity are classified as neuroactive steroids (NAS). Animal models demonstrate modulation of seizure activity by the NAS 17[unreadable]-estradiol (EST), progesterone (PROG), and allopregnanolone (ALLO). In women, fluctuations in these NAS have been implicated in seizure control in the non-pregnant state, with worsening seizures at certain times of the menstrual cycle (catamenial epilepsy). Human studies have demonstrated an increase in seizure frequency with elevated EST/PROG ratios and with declining or low PROG levels. This has not been studied during pregnancy in women with epilepsy. This proposed study will utilize serum samples (n=810 samples) already collected from 135 women with epilepsy during different stages of pregnancy during a Specialized Center of Research in Women and Gender Issues program project grant. These women were enrolled prospectively with tracking of seizures and medications. Collection of plasma samples occurred at multiple points in each trimester. Based on variable points of enrollment (<20 weeks gestation), we have increased observations/samples in the later trimesters of pregnancy. Seizure frequency will be analyzed during the second and third trimesters of pregnancy and compared to the nonpregnant baseline for each subject. Consistent with the R03 mechanism, the current application will extend the analysis of these existing data/samples via measurement of the neuroactive steroids EST, PROG, and ALLO. The working hypotheses are 1) during pregnancy, changing concentrations of EST and PROG influence seizure control;2) the progesterone metabolite, ALLO, mediates the seizure-reducing effect of PROG. The following will be analyzed in relationship to change in seizure frequency during pregnancy: EST/PROG ratio, the rate of rise of PROG, and the rate of rise of ALLO. Additionally, given that labor and delivery is a particularly vulnerable time for increased seizures;ALLO and PROG levels will be compared between women who had peripartum seizures and those who did not. This study can ultimately lead to a better understanding of the NAS regulation of seizure control during pregnancy. Insights gained from this study could provide the impetus for further development of NAS analogs, with treatment benefits extending to both genders and across all ages. During pregnancy, treatment with supplemental progesterone could allow for decreased levels of fetal exposure to AEDs in utero, with improved seizure control and reduced anatomical and neurodevelopmental teratogenicity. PUBLIC HEALTH RELEVANCE: Treating women with epilepsy during pregnancy requires a precarious balance of controlling the mother's seizures without exposing the developing fetus to more anticonvulsant medication than necessary. During pregnancy, the rising sex steroid hormones and their metabolic byproducts may directly influence seizure control;this study will analyze blood samples already obtained in women with epilepsy during pregnancy to examine whether women with increased seizures have alterations in neuroactive steroid levels. These findings could lead to the development of novel treatment strategies for women with epilepsy during pregnancy, such as use of supplemental progesterone, to improve mother and child health outcomes.